PURPOSE: To prevent release of sheet even by use for many hours at a high temperature and to retain a service life of a heat insulating material by fixing a coating sheet to a carbonaceous molded heat insulating material with a carbonaceous pin.

CONSTITUTION: A carbonaceous molded heat insulating material comprising carbon fibers and/or a thermosetting resin as a main base is coated with a carbonaceous sheet, preferably one containing ≥5wt.% Si and ≥0.5wt.% B. In the operation, the coating sheet is fixed to the carbonaceous molded heat insulating material with a carbonaceous pin composed of isotropic carbon, etc. Consequently, the sheet will not release from the carbonaceous molded heat insulating material even by use at high temperature for many hours to give a heat insulating material having excellent durability.